Ozone: A New Water Management Paradigm

Producing and sanitizing pharmaceutical water isn’t easy — but there are options for doing both less expensively

By Erika Hanley-Onken, MKS Instruments Inc.

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Access to clean water is one of the most important factors considered in selection and development of a new manufacturing facility. Without readily available appropriately clean water, processing would be nearly impossible, changeover and cleaning of reusable equipment would cease, and overall production would grind to a stop.

For example, in the United States, incoming water into any manufacturing facility must meet a minimum standard of EPA drinking water. This municipally treated water is only a first step to the processing required by the pharmaceutical industry to bring this water up to pharmacopoeia standards for Purified Water (PW) per USP <1231>. Commonly used technologies to manufacture PW and other compendial waters include softening, reverse osmosis (RO), deionization (DI), dechlorination, (ultra)filtration and even distillation.

A comprehensive water management plan must address all the water in the facility, including its treatment, storage, delivery and handling. Part of this control plan requires understanding the needs for both periodic and continuous sanitization to mitigate and prevent the buildup of contaminants such as biofilm within the water system.

What is ozone and what does it do?
Among the alternatives available for water sanitization, ozone is recognized as an excellent option for disinfecting biopharmaceutical water systems. Ozone (O3) is a highly reactive molecule made up of three atoms of oxygen, which decays back to oxygen. Care must be taken because ozone is also a toxic gas characterized by a strong, pungent smell.

Ozone occurs naturally but can be generated. The two most common generation methods for pharmaceutical use include silent discharge generation and electrolytic generation. Electrolytic ozone is created when high voltage passes between parallel metal electrodes and through a liquid containing oxygen, e.g., the process water. Electrolytically generated ozone is often used for small quantities of ozone or lower flows. Ozone generated by silent discharge is created when a current-controlled electric discharge is released between high purity refractory metal electrodes into a gas containing oxygen. The ozone is then injected into the water system. Ozone generated by silent discharge provides greater flexibility in terms of controlling ozone concentrations to the amount needed, as it is only limited by the oxygen content of the air rather than by the oxygen usable from the water.

The two methods have different maintenance requirements. Silent discharge generators do not directly touch the water path and generally require little standard maintenance. Electrolytic generators are in direct contact with the process water and will require an isolation system to remove it from the water flow for service. Maintenance recommendations from the manufacturers should be followed to provide the greatest uptime possible.

Prior industry concern is that ozone — specifically that generated by silent discharge — may be considered an “added substance,” as it is introduced into the process water vs. created from it. According to the "added substances" discussion in USP <1231>, every added substance must be removed with appropriate means. As long as it is removed, ozone is not considered an added substance in the production of pharmaceutical grade water. Ozone is often removed via a destruct mechanism such as 254-nm ultraviolet (UV) light.

As ozone is the same no matter how it is generated, with the same standard CAS Registry Number 10028-15-6, and because, for risk mitigation, most ozone systems already have a UV destruct included, this “added substance” concern is already addressed and can be considered slightly outdated. High purity silent discharge generation ozone systems are available with excellent operating history in other highly demanding industries (e.g., semiconductor) and are the standard for mitigating the high levels of contamination in wastewater treatment.

Figure 1: Ozone manufactured by silent discharge method

Ozone Addresses Sanitization
Ozone is used as an alternative to heat sanitization with hot water and steam or chemical disinfection using chlorine, chlorides, peroxides and other chemicals. It is one of the strongest commercially available oxidants, with a disinfecting strength 3,000 times that of chlorine due to its high eV potential. Ozone effectively kills bacteria, viruses, yeast, fungi and other microbes as a function of time, susceptibility of the target organisms (action), ozone concentration and water temperature.

While there are a number of potential sources of contamination in storage and delivery systems for purified and sterile water, one of the most common problems facing purified water (PW) production and delivery is the prevention and removal of biofilms. First described in the mid-1930s, biofilms form on any surface wherever surface-associated microbes are present. Biofilms occur in a wide variety of systems, but are endemic to water systems in the biopharmaceutical industry.

Biofilms are difficult to remove from the wetted surfaces of PW systems for several reasons. The most prevalent of these reasons is the tough surface polymeric exopolysacharrides (EPS) shell biofilms excrete. The EPS provides a matrix in which nutrients are retained and microbes can thrive, thereby continuously contaminating the PW storage and distribution system. Biofilm removal treatments must not only penetrate the EPS but must also be biocidal to the heterogenous microbial population below it. Partial removal of the biofilm’s EPS matrix liberates the underlying colonizing microbes. Unless destroyed, these underlying microbes will migrate and reestablish at new sites, maintaining the contamination of the water system.

Addressing biofilms is thus critical for the maintenance of high-purity water quality. Periodic and continuous sanitizations are options to mitigate biofilm growth and proliferation. However, targeted destruction of biofilms is difficult as there are not many non-specific treatments that can address biofilms’ inherent compositional variability.

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